Abstract
The emergence of heterogeneous populations of methicillin-resistant Staphylococcus aureus (MRSA) causes major problems in routine screening for MRSA. Cefoxitin is a potent inducer of the mecA regulatory system and can be use for detection of heterogeneous populations of MRSA. Detection of the mecA gene by PCR was considered to be the “gold standard”. In this study we determined the sensitivity and specificity of the oxacillin disk diffusion test, cefoxitin disk diffusion test and oxacillin agar screening for detection of MRSA. A total of 124 non-duplicate isolates of S. aureus were included in the study. Methicillin resistance was measured using oxacillin (1μg) and cefoxitin(30μg) disc diffusion method and oxacillin agar screening test (6 mg/ml oxacillin) according to CLSI guideline and PCR for the mecA gene. Compared with the molecular detection of methicillin resistance the overall sensitivities and specificities of the phenotypic tests for cefoxitin disc diffusion were 100%, for oxacillin disc diffusion were 91.7 and 92.8% and for oxacillin agar screening were 95 and 95.5%, respectively. We concluded that in the absence of availability of molecular biology techniques, the cefoxitin disc was the best detector of methicillin resistance in S. aureus related to the other phenotypic tests. Key words: Methicillin-resistant Staphylococcus aureus, mecA, Cefoxitin, polymerized chain reaction
Highlights
Staphylococcus aureus is one of the most common organisms that recovered from clinical bacterial isolates in our area, like other parts of the world (Khorvash et al, 2008a,b; Mohammadtaheri et al, 2010)
We evaluated the methicillin resistance of S. aureus clinical isolates using oxacillin and cefoxitin disk diffusion method and oxacillin agar screening in comparison with mecA-based PCR
Sixty isolates (48.4%) were resistant to methicillin based on cefoxitin disk diffusion test and all of them were positive for mecA gene by PCR method
Summary
Staphylococcus aureus is one of the most common organisms that recovered from clinical bacterial isolates in our area, like other parts of the world (Khorvash et al, 2008a,b; Mohammadtaheri et al, 2010). The major problem in routine screening for MRSA is the heterogeneous population of MRSA In these population, a set of cells show low-level resistance to oxacillin with MIC ranging between 1 to 100 mg/l, while on homogeneous MRSA, the MIC for all of the bacteria is >100 mg/l (Cauwelier et al, 2004). Cefoxitin is considered to better than oxacillin for the detection of heterogeneous MRSA and is recommended by Clinical and Laboratory Standards Institute (CLSI) (Fernandes et al, 2005; Frigatto et al, 2005; Swenson et al, 2005). We evaluated the methicillin resistance of S. aureus clinical isolates using oxacillin and cefoxitin disk diffusion method and oxacillin agar screening in comparison with mecA-based PCR
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